The selection of a Diphtheria Antitoxin to serve as a replacement master standard was made. The antitoxin potency is being determined. This involves the determination of the total units per vial and the appropriateness of the preparation to serve in the following areas: 1) antitoxin potency testing in animals, 2) in vitro potency (Vero cell test in microtiter plates), and 3) rabbit skin testing.